Interface-dominated mechanical behavior of CF/PEKK composites according to different heating rate during thermoforming process

While thermoplastic polymers exhibit several desirable properties, their applicability is limited by their high viscosity and extreme processing conditions. To overcome these limitations, in this study, we used the thermoforming process to produce carbon fiber/polyetherketoneketone (CF/PEKK) laminates, which were pre-made through an oven-based consolidation process using prepregs. The laminates were produced at three different heating rates ($40^{\circ }$C/min, $75^{\circ }$C/min and $100^{\circ }$C/min). The laminates produced at the heating rate of $40^{\circ }$C/min showed improved interlaminar shear strength, $90^{\circ }$ tensile strength, and average interlaminar fracture toughness. On the other hand, heating at rates higher than $40^{\circ }$C/min increased the initiation value of the interlaminar fracture toughness ($G_{\mathrm{\text{IC}}-\mathrm{\text{I}}})$ but resulted in nonuniform composites of poor quality. This is because increasing the heating rate reduced the uniformity of the heat distribution with the laminates, resulting in the polymer molecules exhibiting different binding rates and thus nonuniform cross-linking. Thus, the proposed method is a suitable one for producing high-quality thermoplastic composites.